1. Field of the Invention
The present invention relates to an image reading apparatus for photoelectrically reading a image of an original or the like.
2. Description of the Prior Art
In a conventional image pickup device, especially an image reading apparatus using a solid state image pickup element such as a charge coupled device or element (to be referred to as a CCD hereinafter), light is irradiated on a surface of an original, and light reflected by the surface is received by the solid state image pickup device. The information received by the image pickup device is electronically scanned along the main scanning direction and by moving a scanning unit with the device along a subscanning direction perpendicular to the main scanning direction. For example, in order to read an original of A3 size with this apparatus under the conditions that the read line density is given as 16 lines/mm and the resolution along the main scanning line is 16 pels/mm, the number of output bits along one main scanning line is 4,752 bits, and the number of main scanning lines is 6,720. However, the number of bits of a commercially available CCD is 2,048. In order to obtain the above number of output bits, three CCDs must be aligned along the main scanning direction. For example, as shown in FIG. 1, an original 4 on an original table 3 is read by a combination of three CCDs 1a to 1c and three photographing lenses 2a to 2c. Each CCD and a corresponding photographing lens can read one-third of the original 4. Reference symbol X denotes a main scanning direction; and Y, a subscanning direction. Reference numeral 12 denotes an image density reference portion to be described later.
In the conventional image reading apparatus using a plurality of image pickup devices (i.e., CCDs), the following problem occurs. A level of an output signal from a circuit coupled to the plurality of CCDs varies in accordance with variations in both CCD sensitivity and in a transmission factor of a near-infrared cut filter. When the output from the reading apparatus is supplied to an output device such as a laser beam printer or an ink jet printer so as to reproduce the image, the image or background densities of portions corresponding to respective CCDs tend to vary.
In an apparatus which allows reading of a color image, color balance is disturbed in the reproduced image due to variations in the transmission factor of a color separation filter, thereby resulting in a poor image. In particular, in a color image reading apparatus having a plurality of image pickup devices (e.g., CCDs), color balance of the color image is disturbed even when a level of an output signal from a circuit coupled to the image pickup devices changes only slightly, thereby degrading the image quality of the reproduced image.
Variations in CCD sensitivity include sensitivity variations due to similar changes in general spectral sensitivity characteristics of the CCD as indicated by the solid line in FIG. 2. In addition to these, further CCD sensitivity variations are caused when spectral sensitivity factors of spectra excluding blue, green, red and near-infrared spectra change relatively. This mostly results in a color balance disturbance in the color image reading apparatus. In addition to these disadvantages, variations in CCD sensitivity over time is also a critical problem.
The near-infrared cut filter has a transmission factor indicated by the dotted line in FIG. 2. This filter serves as a sensitivity/color correction filter for white exposure and as a color separation filter for improving the color separation by limiting near-infrared transmission of the color separation filter with respect to the CCD. A multi-coating interference filter or a glass filter is used as the near-infrared cut filter. However, both multi-coating filters and glass filters have variations in near-infrared ray wavelengths and transmission factors of visible rays. Similarly, a tricolor (blue, green and red) separation filter has variations in sharp cut wavelengths and transmission factors of the blue, green and red rays. Therefore, the image quality of the reproduced color image is adversely affected by the variations of the filters in addition to the variations in CCD sensitivity factors. Furthermore, variations in electrical characteristics of an amplifier connected to each CCD cannot be neglected as another factor which degrades the quality of the reproduced image.